1. Field of the Invention
This invention relates to a tunable laser in which a plurality of spectral lines different from one another not only in wavelength but also in polarization can be simultaneously oscillated.
2. Description of the Prior Art
Since the advent of the laser, various proposals have been made in regard to applications of the laser. The laser is especially advantageous over light sources of other kinds in that it provides a spectroscopic light source emitting a light beam of satisfactory directivity and the light beam emitted therefrom is excellent in monochromaticity.
However, the laser has a disadvantage that individual sources of laser beams must be prepared for a plurality of desired wavelengths due to the fact that the oscillation wavelength of the laser beam cannot be freely controlled. Such a disadvantage is obviated by wavelength conversion techniques including dye laser and parametric oscillator.
An atomic absorption spectroscopic analysis for the quantitative analysis of an element contained in a sample comprises causing a spectral line having the same wavelength as that of the specific element to pass through an atomic vapor layer of the element and measuring the quantity of light absorption occurring when the atoms in the ground state in the atomic vapor of the element are excited to shift to an upper state. The detection sensitivity of the atomic absorption spectroscopic analysis can be improved when the emission spectral line width of the light source used in the analysis is selected to be at least equal to the absorption spectral line width of the element which is quantitatively analysed. In a prior art technique of atomic absorption spectroscopic analysis, emission from a hollow cathode discharge tube or high frequency discharge tube containing the same element as that quantitatively analysed has been utilized for such a purpose. However, the prior art analysis technique has been inconvenient in that separate light sources must be prepared when it is desired to quantitatively analyze a plurality of elements.
Further, two-wavelength measurement is frequently employed in the technique of atomic absorption spectroscopic analysis. This two-wavelength measurement is employed as a compensating means which eliminates such a disadvantage that the precision of measurement in the prior art atomic absorption spectroscopic analysis technique tends to be adversely affected by variations or instability of the emission intensity of the light source and the concentration of the sample relative to time. A method disclosed in an article of T. Hadeishi reported in "Applied Physics Letters", Vol. 21, pp. 438-440 (1972) is a typical example of the two-wavelength measurement and employs a Zeeman-effect tunable light source. According to the method disclosed in this article, a light source (commonly containing an isotope of a sample element subjected to quantitative analysis) is used so as to emit a suitable spectral line which is very close to the absorption spectral line of the element to be quantitatively analysed, and a magnetic field is applied to the light source for the purpose of Zeeman splitting of the emitted spectral line. One of the components obtained by the Zeeman splitting is subjected to the Zeeman shift so that it has the same wavelength as that of the absorption spectral line of the sample element, while the other component is utilized as reference light which is not absorbed by the sample element. These two components can be easily separated from each other by an analyser due to the fact that they are polarized in different degrees by the Zeeman effect. Thus, this method is advantageous in that, when the light emitted from the light source is passed through the sample and the intensities of the two polarization components contained in the light transmitted through the sample are compared with each other, the ratio therebetween indicates solely the quantity of light absorbed by the sample and the result of analysis is not adversely affected by variations or instability of the emission intensity of the light source and the concentration of the sample. However, the Zeeman-effect tunable light source is defective in that the tunable range of the wavelength is quite narrow. Because of this defect, it is also inevitable to prepare separate light sources for the analysis of a plurality of elements.